Single phase yttrium phosphate having the xenotime crystal structure and method for its synthesis

ABSTRACT

Methods for producing substantially single phase yttrium phosphate which exhibits the xenotime crystal structure are disclosed. The methods can be practiced without the use of high temperatures (e.g., the methods can be practiced at temperatures less than 1000° C.). The resulting yttrium phosphate can be in the form of particles which comprise interwoven strands of crystals of yttrium phosphate and/or nanoparticles prepared from such particles.

CROSS REFERENCE TO RELATED APPLICATION

This is a divisional of U.S. patent application Ser. No. 12/303,658filed on Jun. 1, 2007, which claims the benefit of priority under 35U.S.C. §365 of International Patent Application Serial No.PCT/US07/012915 filed on Jun. 1, 2007 designating the United States ofAmerica, and which claims the benefit under 35 USC §119(e) of U.S.Provisional Application No. 60/811,222 filed Jun. 5, 2006, the contentsof which in its entirety are hereby incorporated by reference.

BACKGROUND OF INVENTION

This invention relates generally to yttrium phosphate and a method forproducing it in commercial quantities, and is particularly concernedwith a pure or single phase yttrium phosphate having the xenotimecrystal structure and a process for synthesizing it without utilizingextremely high temperatures.

In recent years many researchers have explored the use of yttriumphosphate in the field of ceramic materials. Yttrium phosphate appearsto be valuable for use in laminate composites, as a fiber-matrixinterface in ceramic matrix composites and as coatings for thermalprotection. It appears to be particularly useful as a coating because ofits resistance to expansion when exposed to high temperatures.

Although the synthesis of yttrium phosphate by various researchers hasbeen reported, either via expensive high-temperature solid statereactions and wet chemical precipitation, large quantities of yttriumphosphate for commercial applications do not appear to be available.Furthermore, it is very important in the field of ceramic materialsprocessing that any yttrium phosphate utilized be free of secondaryphases and other impurities. Thus, there is a distinct need for thedevelopment of relatively inexpensive methods to synthesize largequantities of pure or single phase yttrium phosphate, especially yttriumphosphate having the xenotime crystal structure.

SUMMARY OF THE INVENTION

In accordance with the invention, it has now been found that pure phaseyttrium phosphate with the xenotime crystal structure can be synthesizedusing a relatively low temperature process that begins by forming aslurry of a solid and relatively insoluble yttrium compound, preferablyyttrium oxide, in water. Phosphoric acid is then added to the slurry inan amount less than the stoichiometric amount required to form yttriumphosphate. Thus, the mole ratio of yttrium to phosphorus in the slurryis greater than 1.0. An inorganic acid, preferably nitric acid, is thenadded to the slurry to react with the excess yttrium compound andthereby form a water-soluble yttrium salt. The solid yttrium phosphateformed by the reaction of the yttrium compound with the phosphoric acid,which is substantially free of any excess phosphoric acid and yttriumcompound, is then removed from the slurry, washed to remove solubleimpurities and dried, usually at temperatures well below 1000° C. Theresultant material is a single or pure phase yttrium phosphate havingthe xenotime crystal structure that is free of unreacted yttriumcompound and phosphoric acid and contains no other forms of yttriumphosphate. The fact that this pure phase yttrium phosphate can be madewithout the need to utilize temperatures above 1000° C. means that theuse of the process of the invention results in substantial cost savings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are XRD patterns. In particular, FIG. 1 is an XRD patternof a sample obtained from an in-progress test after phosphoric acidaddition, then filtered, washed and dried at 1,000° C., while FIG. 2shows the transformation to single phase yttrium phosphate afterreaction with mineral acid, washing and drying to 1,000° C.

FIGS. 3 and 4 are images of Thermogravametric (TGA) and DifferentialScanning calorimetry tests performed on the same samples as FIGS. 1 and2.

DETAILED DESCRIPTION OF THE INVENTION

The first step in the process of the invention for producing commercialamounts of yttrium phosphate is the formation of a slurry containing ayttrium compound. Any solid yttrium compound that is relativelyinsoluble in water can be used. Typically, the yttrium compound willhave a solubility less than about 0.1 grams/liter. Examples of yttriumcompounds that can be used include yttrium oxide, yttrium carbonate,yttrium bicarbonate and hydroxide. Generally, enough of the solidyttrium compound is mixed with water so that the resultant slurrycontains between about 0.5 and 50 weight percent solids, preferablybetween about 3.0 and about 20 weight percent, and more preferablybetween about 5 and 15 weight percent.

While the aqueous slurry of the yttrium compound is vigorously agitated,phosphoric acid is added to form yttrium phosphate. It has been foundthat using a stoichiometric amount of phosphoric acid results in theformation of yttrium phosphate containing unreacted yttrium compound andunreacted phosphoric acid as well as non-xenotime yttrium phosphate. Itis theorized that this contaminated yttrium phosphate results from anincomplete reaction due to the encapsulation of unreacted yttriumcompound and the slow disassociation of phosphoric acid. It has beensurprisingly found that limiting the amount of the phosphoric acid thatis added to less than the stoichiometric amount needed ultimatelyresults in the formation of a pure single phase yttrium phosphate withthe xenotime mineral structure. Thus, the phosphoric acid is added tothe aqueous slurry of the yttrium compound in an amount that is lessthan the stoichiometric amount required for the formation of yttriumphosphate.

Generally, a 75 to 85 weight percent solution of phosphoric acid isadded to the slurry over a period of about 15 minutes to about 90minutes as the slurry is continuously agitated and maintained at atemperature that typically ranges between about 20° C. and about 70° C.The amount of the phosphoric acid added to the slurry is usually about1.5 molar percent less than the amount of the yttrium compound presentin the slurry. When the yttrium compound used is yttrium oxide, thereaction takes place to form yttrium phosphate, minor amounts of yttriumoxide, minor amounts of surface adsorbed phosphoric acid and water. Theyttrium phosphate formed comprises approximately 95.5 percent of thesolids portion in the slurry.

In order to remove the excess yttrium oxide and phosphoric acid from thesolids in the slurry, a small amount of an inorganic acid is added tothe slurry. The acid releases the yttrium compound and phosphoric acidfrom the yttrium phosphate and allows them to react on their own untilthe phosphoric acid is consumed. The 1.5 molar percent excess of theyttrium compound reacts with the acid to form a soluble yttrium saltwhich dissolves in the aqueous phase of the slurry. When yttrium oxideis used as the yttrium compound and nitric acid is used as the inorganicacid, the reaction provides soluble yttrium nitrate which combines withthe remaining phosphoric acid to produce yttrium phosphate solids andwith minor amounts of yttrium nitrate in solution.

The yttrium phosphate, which at this point in the process has thecrystal structure of the mineral churchite (hydrated yttrium phosphate),is then separated from the aqueous phase by filtration, centrifugationor other liquid-solids separation technique.

Although nitric acid is used for purposes of illustration, many otherinorganic acids can normally be utilized to solubilize the excessyttrium compound to remove it from the precipitated yttrium phosphate.Examples of such acids include hydrobromic acid, hydroiodic acid, andsulfuric acid.

Once the yttrium phosphate is removed from the aqueous phase of theslurry, it is normally washed with water to remove any residual solubleimpurities and then dried at temperatures below 1000° C. It has beenfound that the yttrium phosphate removed from the aqueous slurry isultra high purity, single phase needle crystals of the mineral churchiteand is essentially free of unreacted constituents and non-churchiteyttrium phosphate. The drying step is only needed to drive off moistureconverting the yttrium phosphate from the churchite to the xenotimecrystal structure and not to decompose unreacted yttrium compound,phosphoric acid, or other impurities. The conversion from churchite toxenotime crystal structure occurs at approximately 300° C. In view ofthis, substantial cost savings can be obtained by drying the yttriumphosphate at relatively low temperatures between about 300° C. and 900°C., preferably at a temperature below 500° C.

The yttrium phosphate recovered from the drying step is substantiallypure single phase yttrium phosphate of the xenotime crystal structure.The molecular formula is Y_(a)PO₄ where a ranges from 1.000 to 1.005.Preferably, the amount of yttrium present does not exceed 0.25 molepercent excess Y based on the formula YPO₄. The particles of the yttriumphosphate formed are needle-like and appear in the form of soft clumpsof interwoven strands of fine crystals. The clumps can be easily spreadapart to form nanosize particles.

The nature and objects of the invention are further illustrated by thefollowing examples, which are provided for illustrative purposes onlyand not to limit the invention as defined by the claims. The examplesshow the effect of using mineral acid to dissolve excess yttrium oxideand form pure phase yttrium phosphate.

FIG. 1, i.e., XRD pattern of sample DW-15-127-1, illustrates the solidmaterial, predominantly yttrium phosphate, with minor amounts of yttriumoxide present. The sample was obtained from an in-progress test afterphosphoric acid addition, then filtered, washed and dried at 1,000° C.

FIG. 2, i.e., XRD pattern DW-15-127-3, shows the transformation frommaterial as shown in DW-15-127-1 to single phase yttrium phosphate afterreaction with mineral acid, washing and drying to 1,000° C.

FIGS. 3 and 4 are images of Thermogravametric (TGA) and DifferentialScanning calorimetry tests performed on the same samples as FIGS. 1 and2. FIGS. 3 and 4 confirm phase purity due to the absence of furtherphase transformations.

Although the invention has been described in detail for the purpose ofillustration, it is understood that such detail is solely for thatpurpose and variations can be made therein by those skilled in the artwithout departing from the spirit and scope of the invention which isdefined by the following claims.

What is claimed is:
 1. A composition comprising substantially singlephase yttrium phosphate compounds which exhibit the xenotime crystalstructure where the compounds satisfy the formula:Y_(a)PO₄ where a ranges from 1.000 to 1.005.
 2. A composition comprisingsubstantially single phase yttrium phosphate compounds which exhibit thexenotime crystal structure where the compounds satisfy the formula:Y_(a)PO₄ where a ranges from 1.000 to 1.0025.
 3. The composition ofclaim 1 wherein the composition is in the form of particles whichcomprise interwoven strands of crystals of the yttrium phosphatecompounds.
 4. Nanosize particles prepared from the composition of claim3.
 5. A composition comprising hydrated yttrium phosphate compoundshaving the crystal structure of the mineral churchite wherein thehydrated yttrium phosphate compounds satisfy the formula:Y_(a)PO₄·nH₂O, where a ranges from 1.000 to 1.005.
 6. A compositioncomprising hydrated yttrium phosphate compounds having the crystalstructure of the mineral churchite wherein the hydrated yttriumphosphate compounds satisfy the formula: Y_(a)PO₄·nH₂O, where a isranges from 1.000 to 1.0025.